Liquid crystals are used, in particular, as dielectrics in display devices since the optical properties of such substances can be effected by an applied voltage. Electrooptical devices based on liquid crystals are extremely well known to those skilled in the art and may be based on various effects. Devices of this type are, for example, cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SBE (super-birefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability toward electrical fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have low viscosity and give short addressing times, low threshold voltages and high contrast in the cells. Furthermore, they should have a suitable mesophase, for example, for the abovementioned cells, a nematic or cholesteric mesophase, at customary operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, the media desired for matrix liquid-crystal displays containing integrated nonlinear elements for switching individual image points (MLC displays) are those having high positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high specific resistance,good UV and temperature stability of the resistance and a low vapor pressure.
Matrix liquid-crystal displays of this type are known. Examples of nonlinear elements which can be used to individually switch the individual image points are active elements (i.e. transistors). This is then referred to as an "active matrix", and a differentiation can be made between two types:
1. MOS (Metal Oxide Semiconductor) or other diodes on a silicon wafer as substrate. PA1 2. Thin-film transistors (TFTs) on a glass plate as substrate. PA1 broadened nematic phase range (in particular down to low temperatures), PA1 switchability at extremely low temperatures (outdoor use, automobiles, avionics), PA1 increased stability to UV radiation (longer life). PA1 A.sup.1 and A.sup.2 are each, independently of one another, a PA1 Z.sup.1 and Z.sup.2 are each, independently of one another, --CO--O--, --O--CO--, --CH.sub.2 O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 --, --CH.dbd.CH--, --C.tbd.C-- or a single bond, and one of the radicals Z.sup.1 and Z.sup.2 is alternatively --(CH.sub.2).sub.4 -- or --CH.dbd.CH--CH.sub.2 CH.sub.2 --, PA1 L.sup.1, L.sup.2 and L.sup.3 are each, independently of one another, H or F, PA1 m is 0, 1 or 2, PA1 n is 0 or 1. PA1 medium additionally contains one or more compounds selected from the group comprising the general formulae II to V: ##STR13## in which the individual radicals are as defined below: R.sup.o : alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having 1 to 7 carbon atoms PA1 X.sup.o : F, Cl, CF.sub.3, OCF.sub.3, OCHF.sub.2, OCH.dbd.CF.sub.2, OCF.dbd.CF.sub.2, OCFH--CF.sub.2 H or OCF.sub.2 --CF.sub.2 H PA1 Y.sup.1 and PA1 Y.sup.2 : each, independently of one another, H or F PA1 r: 0 or 1. PA1 The proportion of compounds of the formulae I to V together is at least 50% by weight in the total mixture PA1 the proportion of compounds of the formula I is from 10 to 50% by weight in the total mixture PA1 the proportion of compounds of the formulae II to V is from 30 to 70% by weight in the total mixture ##STR16## the medium contains compounds of the formulae II, III, IV or V R.sup.o is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms PA1 the medium consists essentially of compounds of the formulae I to V PA1 the medium contains further compounds, preferably selected from the following group comprising the general formulae XII to XVI: ##STR17## in which R.sup.o and X.sup.o are as defined above and the 1,4-phenylene rings may be substituted by CN, chlorine or fluorine. The 1,4-phenylene rings are preferably mono- or polysubstituted by fluorine atoms. PA1 the I: (II+III+IV+V) weight ratio is preferably from 1:10 to 10:1. PA1 medium consists essentially of compounds selected from the group comprising the general formulae I to XI. PA1 Z.sup.1' and Z.sup.2' are each, independently of one another, --CH.sub.2 CH.sub.2 --, --C.tbd.C--, --CO--O--, --O--CO-- or a single bond, and PA1 n is 0, 1 or 2. PA1 DMEU 1,3-dimethyl-2-imidazolidinone PA1 POT potassium tertiary-butoxide PA1 THF tetrahydrofuran PA1 pTSOH p-toluenesulphonic acid
The use of monocrystalline silicon as the substrate material limits the display size since even the modular assembly of the various part displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electrooptical effect used is usually the TN effect. A differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries the transparent counterelectrode. Compared with the size of the image point electrode, the TFT is very small and hardly affects the image at all. This technology can also be extended to fully color-compatible image displays, where a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable image element.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC display here covers any matrix display containing integrated nonlinear elements, i.e. in addition to the active matrix, also displays containing passive elements such as varistors or diodes (MIM =metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. In addition to problems with respect to the angle dependency of the contrast and the switching times, problems result in MLC displays due to inadequate specific resistance of the liquid-crystal mixtures TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288, Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff., Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff., Paris. As the resistance decreases, the contrast of an MLC display worsens and the problem of "afterimage elimination" may occur. Since the specific resistance of the liquid-crystal mixture generally decreases over the life of an MLC display due to interactions with the internal surfaces of the display, a high (initial) resistance is very important to give acceptable service lives. In particular in the case of low-voltage mixtures, it was hitherto not possible to achieve very high specific resistances. It is furthermore important that the specific resistance increases as little as possible with increasing temperature and after heating and/or exposure to UV radiation. The low-temperature properties of the mixtures from the prior art are also particularly disadvantageous. It is required that crystallization and/or smectic phases do not occur, even at low temperatures, and that the temperature dependence of the viscosity is as low as possible. The MLC displays of the prior art thus do not satisfy current demands.
Thus, there continues to be a great demand for MLC displays of very high specific resistance and at the same time a broad operating temperature range, short switching times, even at low temperatures and low threshold voltage which do not have these disadvantages or only do so to a lesser extent.
For TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
The media available from the prior art do not make it possible to achieve these advantages whilst simultaneously retaining the other parameters.
For supertwisted (STN) cells, media are desired which have a greater multiplexing ability and/or lower threshold voltages and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further extension of the parameter latitude available (clearing point, smectic-nematic transition or melting point, viscosity, dielectric values, elastic values) is urgently desired.